Flash memory is known for the advantages of providing short access time and superior vibration-proof capability. Generally, flash memory can be of either NOR or NAND type. Compared with NOR flash memory, NAND flash memory has greater storage density and storage capacity, and higher sequential access speed, which makes it more suitable for mass storage devices.
A NAND memory device typically includes multiple blocks of memory cells, each block having an equal number of pages, and each page having an equal number of columns of cells. Each page may include some spare columns to increase reliability of the device, in addition to the main columns used for recording data. Herein, main columns and spare columns are collectively referred to as “data columns.”
A NAND flash device may also include redundant columns used for defect repair. For example, if a data column is determined to be defective during a manufacturing test, a repair process may be performed to disable the defective data column and activate a redundant column to replace it. In NAND flash devices, data is usually read/written on a page-wise basis, and erased on a block-wise basis. Therefore, even if only a fraction of memory cells in a block or page are defective, the entire block has to be declared as unusable until it is repaired.
Typically an NAND flash device has only a limited number of redundant columns due to space limitations of the device. Therefore, if the number of defective data columns is greater than the number of redundant columns, it has to be determined which defective data columns are to be disabled and replaced by the redundant columns in the repair process.